CDMA is a signal modulation technique used in a variety of applications, such as cellular and wireless communications systems. In such systems, multiple users communicate at will with a base station over a common frequency band, with each user transmitting a uniquely coded signal. Consequently, the received signal at the base station is a composite of many differently coded signals.
At each user's transmitter, that user's coded signal is formed using a sequence of code coefficients in either of two ways. In nondifferentially encoded CDMA systems, each user's symbol is multiplied by a sequence of code coefficients. In differentially coded systems, instead of multiplying each user symbol by a code sequence, the difference between certain symbols and preceding symbols is multiplied by the sequence of code coefficients. In either arrangement, the multiplication process is known as spreading since the signal spectrum of the coded output symbols extends across a wider frequency range than that of the uncoded user symbols. At the receiver, each user's encoded digital symbols are recovered from the incoming composite signal using a signal despreader. In the despreader, each user's symbols are recovered by multiplying the incoming composite signal with an associated one of a plurality of different code coefficient sequences. In the prior art, the code sequence associated with each user is a replica of the code sequence used to encode that user's symbols.
It has long been recognized that during transmission a substantial amount of interference can be introduced into each coded signal from the other coded signals and compensation for this interference must be provided for intelligible communications. To reduce this interference, a number of different interference reduction techniques have been devised. In one prior art technique, apparatus is used in the receiver which operates on each user's symbols outputted by a despreader using priorly recovered other users' symbols. See, for example, U.S. Pat. No. 5,136,612, issued Aug. 4, 1992 and entitled "Method and Apparatus for Reducing Effects of Multiple Access Interference in a Radio Receiver in a Code Division Multiple Access Communications System". Another class of prior art systems uses an approach which operates on the received composite signal over a time interval using blocks of code coefficients wherein each block includes the code coefficients of each user corresponding to this time interval. See, for example, a publication entitled "Near-Far Resistance of Multiuser Detectors in Asynchronous Channels", I.E.E.E. Transactions on Communications, Vol. 38, No. 4, April 1990, and, more recently, a pending patent application entitled Data Recovery Technique for Asynchronous CDMA Systems, Ser. No. 982,168, filed on Nov. 24, 1992, and assigned to the present assignee. The potential shortcoming with all of these prior art arrangements is that they require the addition of interference canceller circuitry in the receiver and thus incur the cost of implementing such circuitry. In addition, they are not readily suitable for improving the performance capabilities of existing CDMA systems on a retrofit basis.
It would, therefore, be desirable if a low-cost, data recovery technique could be developed for CDMA systems which can be readily added to existing systems.